The invention described herein was made in the course of, or under, a contract with the U.S. Energy Research and Development Administration (ERDA), the successor in interest to the U.S. Atomic Energy Commission (AEC).
1. Field of the Invention
This invention relates to a method for the generation of power and also to a thermal power plant for utilization of the method. More particularly, it relates to a vaporizable fluid cycle, typically a steam cycle, for a liquid metal cooled nuclear reactor, which minimizes temperature differences between working fluids in the steam generator, and provides continued flow from an available inventory of heated fluid to minimize thermal transients during startup and upon loss of normal feed fluid flow, the inventory of heated fluid having a chemical composition similar to the normal feed fluid.
2. Description of the Prior Art
Liquid metal cooled fast breeder nuclear reactors typically include three fluid circuits to achieve the generation of electrical power. The first, or primary fluid circuit, circulates a liquid metal, such as sodium, which removes heat generated in the reactor core and transfers it, through a heat exchanger, to an intermediate fluid in the second circuit. The intermediate fluid is typically similar to the primary fluid, and transfers heat to a vaporizable fluid in a utilization circuit, typically to water in a steam cycle. The main component in which the fluid is vaporized by heat from the intermediate circuit is referred to as a steam generator.
In certain systems the steam generator is desired to be of a "once-through" type; that is, there is no recirculation of the utilization fluid in an evaporator or drum component. The utilization fluid, subsequent to condensation in the turbine-generator condenser, passes through a series of heating stages, enters the steam generator and is then evaporated and most often times superheated. Superheating may take place in the steam generator or in a separate unit. The fluid then passes to the turbine-generator and condenser, completing the circuit.
Two major concerns experienced in operation of such steam generators in the prior art are (1) thermal stresses induced by temperature differences between the intermediate and utilization fluids, and (2) the necessity to provide an auxiliary source of utilization fluid and means for its injection if the steam generators are to be used for decay heat removal, that is, the heat which continues to be released after the reactor has been shut down or is in hot standby.
An alternate method for providing continued cooling has been to provide one or more circuits for cooling of primary or secondary sodium systems with air. A further general alternate has been to use a recirculating steam generator, with natural or forced circulation, and a steam drum containing a significant inventory of boiler water. This permits continued steam generation and cooling after loss of feed fluid flow, causing minimum temperature changes and providing time for initiation of alternate auxiliary cooling systems. Because of the rapid decrease with time of the heat release rate, such auxiliary systems then can be designed for lower maximum capabilities. The disadvantage of this design is the consequent increased requirements in the size of the evaporator sections of the steam generators to permit the high recirculation ratio required for adequate heat transfer with the recirculated water. It is characteristic of recirculating steam generators that impurities in the feedwater are accumulated in the evaporator section and concentrations are limited only by blowdown of boiler fluid or carry over to the superheater. If, in the interests of economy of first cost, the recirculation ratio, and hence evaporator size, is decreased, a situation is achieved where heat transfer conditions correspond more to once-through operation, but at the higher concentration of feedwater impurities characteristic of recirculating units. To limit concentration of impurities to more acceptable levels, very high blowdown rates from the steam generator are required.
Further, whether a once-through or recirculation type is utilized, upon accident conditions such as loss of normal feed fluid flow, the auxiliary fluid must be immediately supplied to the steam generator to remove heat from the intermediate fluid, and hence the primary fluid, during the time period necessary to effect a controlled shutdown of the reactor. In addition to the above, many prior art systems have provided an additional available source of auxiliary fluid. However, this source is typically a large tank of fluid which is available for other plant functions as well, and is neither heated as is the normal feed fluid, nor is the chemistry controlled as finely as the normal feed fluid chemistry. Therefore, immediate injection of the auxiliary fluid may induce severe thermal stresses at the inlet and along the steam generator. Similarly, thermal stresses are also induced at the steam generator feed fluid inlet during startup conditions.
It is therefore highly desirable to provide a means whereby, during plant startup and during the initial stages of a loss of normal feed fluid incident when the necessary heat removal rate is greatest, that fluid with temperature and chemical properties identical to the normal feed fluid be provided to the steam generator. It is further desirable to provide a means for controlling the chemical properties of the feed fluid during normal and accident operation, to minimize the potential for chemical buildup in the steam generator. This invention provides such means, thereby minimizing the potential for thermal shock and impurity buildup in the steam generator.